The disclosure relates to a laser apparatus.
With miniaturization and high integration of a semiconductor integrated circuit, an improvement in resolution has been demanded in recent years for a semiconductor exposure apparatus. Hereinafter, the semiconductor exposure apparatus is simply referred to as an “exposure apparatus”. Shortening in a wavelength of light to be outputted from an exposure light source has been in progress accordingly. In general, a gas laser apparatus is used in place of an existing mercury lamp for the exposure light source. For example, a KrF excimer laser apparatus and an ArF excimer laser apparatus each may be used as a gas laser apparatus directed to exposure. The KrF excimer laser apparatus may output ultraviolet laser light of a wavelength of 248 nm, and an ArF excimer laser apparatus may output ultraviolet laser light of a wavelength of 193 nm. For example, reference is made to Japanese Patent No. 4290001, U.S. Pat. No. 6,693,939, Japanese Patent No. 4818871, Japanese Patent No. 5096035, Japanese Patent No. 5111930, and Japanese Patent No. 5178047.
As a next generation exposure technology, liquid immersion exposure has been brought into practical use in which a clearance between an exposure lens on exposure apparatus side and a wafer is filled with a liquid. The liquid immersion exposure allows for a change in a refractive index between the exposure lens and the wafer, making it possible to shorten an apparent wavelength of light derived from the exposure light source. When the liquid immersion exposure is performed using the ArF excimer laser apparatus as the exposure light source, ultraviolet rays with a wavelength of 134 nm in water may be applied to the wafer. This technology is referred to as ArF liquid immersion exposure. The ArF liquid immersion exposure may also be referred to as an ArF liquid immersion lithography.
On the other hand, a spectral line width in free oscillation of each of the KrF excimer laser apparatus and the ArF excimer laser apparatus is wide, e.g., in a range from about 350 pm to about 400 pm. This may result in color aberration and its consequential decrease in resolution when a projection lens is made of a material that allows for transmission of ultraviolet rays including the KrF laser light and the ArF laser light. It is therefore necessary to narrow the spectral line width of the laser light to be outputted from the gas laser apparatus to an extent in which the color aberration is negligible. To narrow the spectral line width, a line narrowing module or “LNM” including a line narrowing device may be provided in a laser resonator of the gas laser apparatus. Non-limiting examples of the line narrowing device may include an etalon and a grating. In the following description, the laser apparatus that allows for narrowing of the spectral line width is referred to as a line narrowing laser apparatus.